detecting protected constructors of (possibly abstract) base class

Question

I am experimenting with the new features of C++11. In my setup I would really love to use inheriting constructors, but unfortunately no compiler implements those yet. Therefore

Answer 1

This appears to work fine on my local GCC (4.7, courtesy of rubenvb). GCC on ideone prints several "implemented" compiler internal errors though.

I had to make the "implementation details" of the Experiment class public, because for some reasons (which smells like a bug), my version of GCC complains about them being private, even though only the class itself uses it.

#include <utility>

template<typename T, typename Ignored>
struct Ignore { typedef T type; };

struct EatAll {
  template<typename ...T>
  EatAll(T&&...) {}
};

template<typename T>
struct Experiment : T {
public:
  typedef char yes[1];
  typedef char no[2];

  static void check1(T const&);
  static void check1(EatAll);

  // if this SFINAE fails, T accepts it
  template<typename ...U>
  static auto check(int, U&&...u)
    -> typename Ignore<no&, 
        decltype(Experiment::check1({std::forward<U>(u)...}))>::type;

  template<typename ...U>
  static yes &check(long, U&&...);

public:
  void f() {}
  template<typename ...U, 
           typename std::enable_if<
             std::is_same<decltype(Experiment::check(0, std::declval<U>()...)),
                          yes&>::value, int>::type = 0>
  Experiment(U &&...u):T{ std::forward<U>(u)... }
  {}
};

// TEST

struct AbstractBase {
  protected:
    AbstractBase(int, float);
    virtual void f() = 0;
};

struct Annoyer { Annoyer(int); };

void x(Experiment<AbstractBase>);
void x(Annoyer);

int main() {
  x({42});
  x({42, 43.f});
}

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Update: The code also works on Clang.